1. Field of the Invention
The present invention relates to a recording apparatus such as a laser beam printer, and, more particularly to an image processing apparatus which receives multi-value signals transmitted from an external apparatus such as a host computer and which is capable of reproducing high quality images including printed halftone images.
2. Related Background Art
Recently, a laser beam printer has become important with the wide use of the DTP, and therefore there is a desire for the laser beam printer to output high quality and high density images. Furthermore, printers capable of expressing halftone images have been suggested recently in the aforesaid state.
FIG. 4 illustrates an image processing portion of a printer capable of expressing halftone images. When 4-bit multivalue image data supplied from external equipment (omitted from illustration) is loaded in a counter 31 in synchronization with the first transition of an image clock, the counter 31 decreases its count in accordance with a count clock transmitted from a count clock generator 32. When the count value is "0", the counter 31 transmits a Carry signal shown in FIG. 5E. The carry signal is supplied to the CLR of a J.K flip-flop 33, and then a signal obtained by pulse width modulation as shown in FIG. 5F is transmitted from an output Q of the J.K flip-flop 33. Then, the signal, the pulse width of which has been modulated, is supplied to a laser driver (omitted from illustration), so that a halftone image is printed by an electrophotography method in which the laser source is turned on/off to expose a photosensitive drum to light. In order to form a 64-gradation image, a clock which is 64 times the image clock in frequency must be used as the counting clock.
However, in such electrophotography, the pulse width and the density of the printed image are not in strict proportion to each other, and the gradient is small in the portions at which the pulse width is small or large, and is small in the intermediate portion. Hence, the density of the half tone image will be formed too roughly with the aforesaid pulse width equally sectioned into 64 portions, and thus the halftone image cannot be formed sufficiently clearly. Another available method in which the frequency of the clock is raised encounters a problem in that the cost cannot be reduced.
Hitherto, the laser beam printer has a laser drive circuit arranged as shown in FIG. 33. Referring to FIG. 33, reference numeral 34 represents a constant current circuit the current value of which can be set in response to a current setting analog signal APC 33 supplied from outside. The supply/cut of the set current to the semiconductor laser 5 is switched by a switch circuit composed of a resistor R80, and transistors Tr80 and Tr81. That is, when the level of image signal VIDEO 32 is low, the electric current flows to the semiconductor laser 5, so that the laser is turned on. When the same is high, the electric current flows between the collector and the emitter of the transistor Tr80 and not to the semiconductor laser.
The laser beam printer uses the laser drive device shown in FIG. 33 to perform, for example, the following printing operation. The laser beam printer has a resolution of 300 dots/inch to draw and print characters and figures with black dots (.circle-solid.) and white dot (.smallcircle.) at positions on a lattice of 300 dot/inch. FIG. 34 illustrates a dot pattern of letter "a".
However, in the resolution of 300 dots/inch according to the aforesaid conventional structure, the interval of the dots is about 85.mu.. Although, in general, it is said that an image detail of a level of about 20.mu. can be distinguished by the human eye, the outline of the character or the figure has notches, and thus the quality of the printed image is unsatisfactory.
In order to overcome the aforesaid problem, use of a method in which the resolution is simply raised will cause the frequency of the control clock required for each electric circuit to be raised. Therefore, a special and expensive device must be used, and in addition, the aforesaid high frequency region is a region in which the laser drive apparatus cannot satisfactorily response to the frequencies.